Camera Exposure

7/29/2019 Camera Exposure

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CAMERA EXPOSURE

A photograph's exposure determines how light or dark an image will appear when it's been capturedby your camera. Believe it or not, this is determined by just three camera settings: aperture, ISO andshutter speed (the "exposure triangle"). Mastering their use is an essential part of developing an

intuition for photography.

UNDERSTANDING EXPOSURE

Achieving the correct exposure is a lot like collecting rain in a bucket. While the rate of rainfall isuncontrollable, three factors remain under your control: the bucket's width, the duration you leave it in

the rain, and the quantity of rain you want to collect. You just need to ensure you don't collect toolittle ("underexposed"), but that you also don't collect too much ("overexposed"). The key is that there

are many different combinations of width, time and quantity that will achieve this. For example, forthe same quantity of water, you can get away with less time in the rain if you pick a bucket that'sreally wide. Alternatively, for the same duration left in the rain, a really narrow bucket can be used as

long as you plan on getting by with less water.

In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to thewidth, time and quantity discussed above. Furthermore, just as the rate of rainfall was beyond your

control above, so too is natural light for a photographer.

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Page 1 of 9Camera Exposure: Aperture, ISO & Shutter Speed

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EXPOSURE TRIANGLE: APERTURE, ISO & SHUTTER

SPEED

Each setting controls exposure differently:

Aperture: controls the area over which light can enter your cameraShutter speed: controls the duration of the exposure

ISO speed: controls the sensitivity of your camera's sensor to a given amount of light

One can therefore use many combinations of the above three settings to achieve the same exposure.The key, however, is knowing which trade-offs to make, since each setting also influences other

image properties. For example, aperture affects depth of field, shutter speed affects motion blur andISO speed affects image noise.

The next few sections will describe how each setting is specified, what it looks like, and how a givencamera exposure mode affects their combination.

SHUTTER SPEED

A camera's shutter determines when the camera sensor will be open or closed to incoming light fromthe camera lens. The shutter speed specifically refers to how long this light is permitted to enter the

camera. "Shutter speed" and "exposure time" refer to the same concept, where a faster shutter speedmeans a shorter exposure time.

By the Numbers. Shutter speed's influence on exposure is perhaps the simplest of the three camera

settings: it correlates exactly 1:1 with the amount of light entering the camera. For example, when theexposure time doubles the amount of light entering the camera doubles. It's also the setting that has

the widest range of possibilities:

Shutter Speed Typical Examples

1 - 30+ seconds Specialty night and low-light photos on a tripod

2 - 1/2 secondTo add a silky look to flowing waterLandscape photos on a tripod for enhanced depth of field

1/2 to 1/30 secondTo add motion blur to the background of a moving subjectCarefully taken hand-held photos with stabilization




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1/50 - 1/100 second Typical hand-held photos without substantial zoom

1/250 - 1/500 secondTo freeze everyday sports/action subject movementHand-held photos with substantial zoom (telephoto lens)

1/1000 - 1/4000 second To freeze extremely fast, up-close subject motion

How it Appears. Shutter speed is a powerful tool for freezing or exaggerating the appearance of

motion:

Slow Shutter Speed

Fast Shutter Speed

With waterfalls and other creative shots, motion blur is sometimes desirable, but for most other shotsthis is avoided. Therefore all one usually cares about with shutter speed is whether it results in a sharp

photo either by freezing movement or because the shot can be taken hand-held without camerashake.

How do you know which shutter speed will provide a sharp hand-held shot? With digital cameras, the

best way to find out is to just experiment and look at the results on your camera's rear LCD screen (atfull zoom). If a properly focused photo comes out blurred, then you'll usually need to either increase

the shutter speed, keep your hands steadier or use a camera tripod.

For more on this topic, see the tutorial on Using Camera Shutter Speed Creatively.

APERTURE SETTING

A camera's aperture setting controls the area over which light can pass through yourcamera lens. It isspecified in terms an f-stop value, which can at times be counterintuitive, because the area of the

opening increases as the f-stop decreases. In photographer slang, when someone says they are




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"stopping down" or "opening up" their lens, they are referring to increasing and decreasing the f-stop

value, respectively.

By the Numbers. Every time the f-stop value halves, the light-collecting area quadruples. There's a

formula for this, but most photographers just memorize the f-stop numbers that correspond to eachdoubling/halving of light:

Aperture Setting Relative Light Example Shutter Speed

f/22 1X 16 seconds

f/16 2X 8 seconds

f/11 4X 4 seconds

f/8.0 8X 2 secondsf/5.6 16X 1 second

f/4.0 32X 1/2 second

f/2.8 64X 1/4 second

f/2.0 128X 1/8 second

f/1.4 256X 1/15 second

The above aperture and shutter speed combinations all result in the same exposure.

Note: Shutter speed values are not always possible in increments of exactly double or half another

shutter speed, but they're always close enough that the difference is negligible.

The above f-stop numbers are all standard options in any camera, although most also allow finer

adjustments, such as f/3.2 and f/6.3. The range of values may also vary from camera to camera (orlens to lens). For example, a compact camera might have an available range of f/2.8 to f/8.0, whereas

a digital SLR camera might have a range of f/1.4 to f/32 with a portrait lens. A narrow aperture rangeusually isn't a big problem, but a greater range does provide for more creative flexibility.

Technical Note: With many lenses, their light-gathering ability is also affected by their transmission

efficiency, although this is almost always much less of a factor than aperture. It's also beyond thephotographer's control. Differences in transmision efficiency are typically more pronounced with

extreme zoom ranges. For example, Canon's 24-105 mm f/4L IS lens gathers perhaps ~10-40% lesslight at f/4 than Canon's similar 24-70 mm f/2.8L lens at f/4 (depending on the focal length).

How it Appears. A camera's aperture setting is what determines a photo's depth of field (the range of

distance over which objects appear in sharp focus). Lower f-stop values correlate with a shallowerdepth of field:




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Wide Aperturef/2.0 - low f-stop number

shallow depth of field

Narrow Aperturef/16 - high f-stop number

large depth of field

ISO SPEED

The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, italso correlates 1:1 with how much the exposure increases or decreases. However, unlike aperture andshutter speed, a lower ISO speed is almost always desirable, since higher ISO speeds dramatically

increase image noise. As a result, ISO speed is usually only increased from its minimum value if thedesired aperture and shutter speed aren't otherwise obtainable.

Low ISO Speed

(low image noise)

High ISO Speed(high image noise)




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note: image noise is also known as "film grain" in traditional film photography

Common ISO speeds include 100, 200, 400 and 800, although many cameras also permit lower orhigher values. With compact cameras, an ISO speed in the range of 50-200 generally producesacceptably low image noise, whereas with digital SLR cameras, a range of 50-800 (or higher) is often

acceptable.

CAMERA EXPOSURE MODES

Most digital cameras have one of the following standardized exposure modes: Auto ( ), Program(P), Aperture Priority (Av), Shutter Priority (Tv), Manual (M) and Bulb (B) mode. Av, Tv, and M are

often called "creative modes" or "auto exposure (AE) modes."

Each of these modes influences how aperture, ISO and shutter speed are chosen for a given exposure.Some modes attempt to pick all three values for you, whereas others let you specify one setting and

the camera picks the other two (if possible). The following charts describe how each mode pertains toexposure:

Exposure Mode How It Works

) Camera automatically selects all exposure settings.

Program (P)

Camera automatically selects aperture & shutter speed; you can choose a

corresponding ISO speed & exposure compensation. With some cameras, P canalso act as a hybrid of the Av & Tv modes.

Aperture

Priority (Av orA)

You specify the aperture & ISO; the camera's metering determines thecorresponding shutter speed.

Shutter Priority

(Tv or S)

You specify the shutter speed & ISO; the camera's metering determines thecorresponding aperture.

Manual (M) You specify the aperture, ISO and shutter speed regardless of whether thesevalues lead to a correct exposure.

Bulb (B)

Useful for exposures longer than 30 seconds. You specify the aperture and ISO;

the shutter speed is determined by a remote release switch, or by the duration untilyou press the shutter button a second time.

In addition, the camera may also have several pre-set modes; the most common include landscape,

portrait, sports and night mode. The symbols used for each mode vary slightly from camera tocamera, but will likely appear similar to those below:




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Exposure Mode How It Works

Portrait Camera tries to pick the lowest f-stop value possible for a given exposure. Thisensures the shallowest possible depth of field.

Landscape Camera tries to pick a high f-stop to ensure a large depth of field. Compactcameras also often set their focus distance to distant objects or infinity.

Sports/Action Camera tries to achieve as fast a shutter speed as possible for a given exposure ideally 1/250 seconds or faster. In addition to using a low f-stop, the fast shutterspeed is usually achieved by increasing the ISO speed more than would otherwise

be acceptable in portrait mode.

Night/Low-light

Camera permits shutter speeds which are longer than ordinarily allowed for hand-

held shots, and increases the ISO speed to near its maximum available value.However, for some cameras this setting means that a flash is used for theforeground, and a long shutter speed and high ISO are used expose the

background. Check your camera's instruction manual for any uniquecharacteristics.

However, keep in mind that most of the above settings rely on the camera's metering system in orderto know what's a proper exposure. For tricky subject matter, metering can often be fooled, so it's agood idea to also be aware of when it might go awry, and what you can do to compensate for such

exposure errors (see section on exposure compensation within the camera metering tutorial).

Finally, some of the above modes may also control camera settings which are unrelated to exposure,

although this varies from camera to camera. Such additional settings might include the autofocuspoints, metering mode and autofocus modes, amongst others.

Want to learn more? Discuss this and other articles in ourdigital photography forums.

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